Kun Han

Cuprate superconductors remain central to condensed matter physics due to their technological relevance and unconventional, incompletely understood electronic behavior. While the canonical phase diagram and low-energy models have been shaped largely by studies of underdoped and moderately doped cuprates, the overdoped regime has received comparatively limited attention. Here, we track the evolution of the electronic structure from optimal to heavy overdoping in La _2−x Sr _x CuO ₄ (LSCO) using broadband optical spectroscopy across x = 0.15–0.60. The measured spectral changes—including the redistribution of Zhang–Rice–related spectral weight—are in qualitative agreement with determinant quantum Monte Carlo (DQMC) simulations of the three-orbital Emery model, which together indicate a pronounced reconstruction of the electronic structure beyond hole concentrations x > 0.2. Guided by these observations, we propose a spontaneous checkerboard-type Zhang–Rice electronic configuration that accounts for the coexistence of itinerant and localized carriers characteristic of the heavily overdoped state. Our results refine the doping-dependent Zhang–Rice–based framework for cuprates, illuminate how correlations persist deep into the overdoped regime, and provide new constraints on microscopic mechanisms of high-temperature superconductivity, with broader implications for correlated transition-metal oxides.

This study demonstrates the design and realization of high-performance, transparent electrostatic energy storage film capacitors by integrating optical functionalities via a nanostructural engineering approach. A series of (1 – x)SrTiO3-xCeO2 [(1 – x)STO-xC, 0.0 ≤ x ≤ 0.5] films were grown on highly transparent (La0.03Ba0.97)SnO3 buffer electrode layers using pulsed laser deposition. The incorporation of CeO2 as a heterogeneous second phase generates lattice-mismatch-associated interfacial strain and structural heterogeneity, thereby transforming the nearly linear dielectric response of STO into a slim nonlinear polarization response with enhanced maximum polarization and low remanent polarization. The (1 – x)STO-xC films with x = 0.4 and 0.5 exhibit exceptional electrostatic energy storage performance, attaining a high recoverable energy density of up to 50 ± 3 J/cm3 and an ultrahigh efficiency of ∼90%. These films simultaneously maintain a high optical transmittance of ∼75% across the visible to near-infrared spectrum with a tunable optical bandgap. These films demonstrate robust frequency reliability from 20 Hz to 5 kHz and thermal stability from 25 to 120 °C. Notably, these lead-free (1 – x)STO-xC films comprise merely four elements (Sr, Ti, O, and Ce). This work establishes a viable strategy for fabricating a simplified-composition, multifunctional film that simultaneously delivers superior energy storage capability and high optical transparency, advancing their integration into next-generation transparent optoelectronic systems.

Relaxor ferroelectric (RFE) films are highly desirable for modern electronic devices and power systems due to their ability to store and recycle electrostatic energy. However, achieving both the high energy density U_e and efficiency η simultaneously remains a significant challenge. Herein, lead-free 0.7BaTiO₃-0.3CeO₂ (BT-C) RFE films with a simple chemical composition (containing only four elements of Ba, Ti, O, and Ce) were developed to modulate energy storage performance via tailoring polarization behavior. The (110)-oriented BT-C films show a high energy storage density U_e of 42.7 J/cm³, with an ultrahigh efficiency η of 93.1% at room temperature, which can be attributed to the improved polarization response and suppressed hysteresis. Moreover, these BT-C films exhibit exceptional frequency stability (2 Hz to 5 kHz) and thermal stability (25-120 °C), along with reliable operation endurance (>10⁶ cycles) without performance degradation. The orientation control strategy, combined with polymorphic nanodomains, offers a promising approach for developing high-performance lead-free energy storage materials, indicating their great potential for practical power-storage applications.

The SrTiO3-based conducting heterointerfaces, when laterally confined by nanopatterning or defined using a conductive atomic force microscope, exhibit superior electronic properties compared to their macroscopic two-dimensional counterparts. This enhancement is attributed to the modulation of nanoscale transport properties by the polarity of SrTiO3 domain walls. However, the impact of polar domain walls on macroscopic electronic behavior remains poorly explored. In this study, we observe two distinct resistance anomalies at approximately 80 and 40 K, characteristic temperatures of SrTiO3 domain wall polarity, by measuring the macroscopic electrical transport properties of the (La0.3Sr0.7)(Al0.65Ta0.35)O3/SrTiO3 conducting heterointerface. As the carrier density decreases with decreasing gating voltage, these anomalies become more pronounced, with a metal-insulator-metal transition occurring near 40 K at a carrier density threshold of approximately 1 × 1012 cm−2. Our findings provide valuable insights into the role of polar domain walls in shaping the electronic characteristics of SrTiO3-based heterostructures and highlight the potential for manipulating macroscopic electronic behavior through domain wall engineering.

Relaxor ferroelectrics are promising for high-performance energy storage applications, particularly in miniaturized electronic devices and power systems. Herein, we investigate the influence of CeO2 doping on the structural and polarization behavior of (1 − x)BaTiO3−xCeO2 [(1 − x)BT−xC, 0.0 ≤ x ≤ 0.5] films. Our results show that the incorporation of CeO2 into BaTiO3 matrix induces chemical and structural heterogeneity, which effectively suppresses hysteresis, achieving a large energy storage density Ue of ∼37.6 J/cm3 and a high efficiency η of ∼80% for 0.7BT-0.3C films grown on Nb-SrTiO3 (001) substrates. The orientation control further refined the energy storage properties, with 0.7BT-0.3C films grown on (110)- and (111)-oriented Nb-SrTiO3 substrates exhibiting improved Ue of 44.8 and 46.8 J/cm3. The frequency and thermal stability analyses revealed that the nanostructured BaTiO3-based films maintained stable energy storage performance across a wide frequency range from 20 Hz to 10 kHz, and temperature range from 25 to 160 °C. These findings suggest the potential of nanostructure engineering and orientation control in improving the energy storage performance of BaTiO3-based films for advanced applications.

The control of interfacial oxygen vacancy provides additional functional tunability to correlated oxide heterostructures. Here, we studied the role of amorphous capping layers, such as LaAlO3 and LaMnO3, in controlling magnetic properties of (La,Sr)MnO3//SrTiO3 heterostructures. When (La,Sr)MnO3 layers are thin (∼4 nm), the amorphous LaAlO3 capping layer significantly suppresses the ferromagnetic interaction, denoted by a decreased Curie temperature TC (ΔTC over −300 K) and saturated moment MS (ΔMS at −2.9 μB/Mn) compared to the uncapped counterparts, irrespective of epitaxial orientations. This ferromagnetic suppression is attributed to the creation of interfacial oxygen vacancies, as evidenced by the recovery of ferromagnetism upon ex situ annealing (1 h at 625 °C in air). Moreover, when replacing LaAlO3 with LaMnO3, the reduction of oxygen affinity in the capping layer results in less pronounced magnetic modulations with ΔTC at −100 K and ΔMS at −1.1 μB/Mn. Also, the increase in (La,Sr)MnO3 layer thickness dilutes the capping-layer effect, leading to negligible changes on TC and MS in the LaAlO3-capped 40-nm-thick (La,Sr)MnO3//SrTiO3 samples. Our results not only shed some light on understanding the capping-layer effect in manganite-based heterostructures but also offer a feasible strategy for functional modulations at other oxide interfaces.

One preferred lead-free ferroelectric, (K,Na)NbO₃, offers prominent features of environmentally benign and excellent piezoelectricity, but suffers from poor ferroelectricity. Mn-doping has improved its electrical properties, yet its site occupancy remained unclear. In this study, Mn-atomic-layered antiphase boundaries were created in (K,Na)NbO₃-based films, revealing Mn occupying the A-site position. These boundaries stabilized ferroelectricity in these (K,Na)NbO₃-based films with a large twice remnant polarization (~72.5 μC/cm²) across a wide frequency range (20 Hz−10 kHz). High-resolution imaging shows densely arranged antiphase boundaries are grown along three crystal axes. These boundaries are Mn-atomic-enriched at a nanoscale width of a single unit cell, equilibrating the interfacial charges and clamp the interfacial strain, resulting in the highly squared hysteresis loops and high Curie temperature of ~400 °C in the films. Our results may provide a paradigm for designing high-performance lead-free ferroelectric films, unleashing their application potential for expelling lead-containing counterparts.

Researchers pursuing advanced photoelectric devices have discovered near room-temperature metal–insulator transitions (MIT) in nonvolatile VO2. Despite theoretical investigations suggesting that polaron dynamics mediate the MIT, direct experimental evidence remains scarce. In this study, we present direct evidence of the polaron state in insulating VO2 through high-resolution spectroscopic ellipsometry measurements and first-principles calculations. We illustrate the complementary role of polaron dynamics in facilitating Peierls and Mott transitions, thereby contributing to the MIT processes. Furthermore, our observations and characterizations of conventional metallic and correlated plasmons in the respective phases of the VO2 film offer valuable insight into their electron structures. This investigation enhances comprehension of the MIT mechanism in correlated systems and underscores the roles of polarons, lattice distortions, and electron correlations in facilitating phase transition processes in strongly correlated systems. Additionally, the detailed detection of small polarons and plasmons serves as inspiration for the development of new device functionalities.

The crucial role of interface engineering in controlling functionality has been well documented in the (001)-orientated perovskite oxide heterostructures; however, limited attention has been given to other epitaxial orientations. In this study, the (La,Sr)MnO3 heterostructure is selected to present the epitaxial-orientation-dependent interface engineering strategies based on magnetoelectric coupling and oxygen octahedral coupling. When capped by a ferroelectric PbTiO3 layer for interlayer magnetoelectric coupling, the (001) (La,Sr)MnO3 layer exhibits the most significant magnetic modulations compared to (110) and (111) counterparts. This can be attributed to the PbTiO3's out-of-plane polarization that depends on the epitaxial orientations. Also, if considering the interfacial oxygen octahedral structure in terms of epitaxial orientations, the buffer (or cap) layer hosts the most part of interfacial oxygen octahedra and, thus, plays a dominant role in controlling oxygen octahedral coupling of (001)-orientated [or (110)-orientated] perovskite heterostructures. This is consistent with our observations that for the (001) [or (110)] (La,Sr)MnO3 heterostructures, the DyScO3 buffer layer offers the more (or less) effective magnetic modulations compared to the cap one. These findings underscore the orientation-dependent nature of interface engineering in manganite heterostructures as well as in other perovskite heterostructures for tailoring functionalities.

Magnetic anisotropy has significant importance in both designing high-efficiency magnetic devices and understanding noncollinear spin textures. Here, La0.67Sr0.33MnO3 layers are epitaxially fabricated on the (001)-, (110)-, and (111)-orientated SrTiO3 substrates to manipulate magnetic anisotropy. While the [11¯0] axis is the in-plane magnetic easy axis for the (001) La0.67Sr0.33MnO3 layer, it becomes the hard axis for the (110) and (111) samples. Interestingly, the (110) samples exhibit a great enhancement in magnetic anisotropy energy, reaching at 6.1 × 105 erg/cm3 that is 1–2 orders of magnitude higher than ones in (001) and (111) films. It is also noted that such improved magnetic anisotropy of (110) samples is maintained up to 30 nm. Those observations can be interpreted in terms of the orientation-modified orbital symmetry. While the energetically favored 3dx2−y2 orbital and degenerated eg orbitals drive the two-dimensional and bulk-like ferromagnetism in (001) and (111) La0.67Sr0.33MnO3 layers, respectively, the 3d3z2−r2 orbital provides the lowest orbital symmetry and results in the highest magnetic anisotropy energy in (110) samples. Meanwhile, this model can also explain the thickness-dependent Curie temperatures with different epitaxial orientations, deepening the understanding of the complex oxide heterostructures for magnetic modulations.